This invention relates to an Automatic Gain Control (AGC) circuit For use in a radio receiving apparatus with a Time Division Multiple Access (TDMA) transmission system.
Such an AGC circuit automatically controls a gain of an amplifier so as to keep an average amplitude of a demodulator input signal at a constant level.
A conventional AGC circuit of the type described comprises a variable gain amplifier for amplifying an input reception signal in accordance with a controllable gain to produce an amplified reception signal, a detector for detecting an envelope of the amplified reception signal to produce a detected signal, a comparator for comparing the detected signal with a predetermined voltage signal to determine a difference therebetween. The difference is supplied to the variable gain amplifier as the controllable gain.
However, since the conventional AGC circuit mainly comprises hardwares, such as the detector and the comparator, unevenness of characteristics cannot be avoided. In addition, the conventional AGC circuit inevitably becomes complicated in construction. Accordingly, a radio receiving apparatus having the conventional AGC circuit therein cannot be fabricated at a low cost.
Furthermore, as processing speed of the input reception signal is under the influence of response speed of the detector and the comparator, it is difficult that the conventional AGC circuit follows up a fast burst signal.